1. Field of the Invention
Embodiments of the invention relate to switching power supplies of a current resonance type, and in particular to switching frequency stabilization of switching power supplies.
2. Description of the Related Art
FIG. 4 shows a circuit diagram of a conventional resonance type switching power supply. The switching power supply comprises a transformer T having a primary winding WP1 and secondary windings WS1 and WS2 with a center tap therebetween in the main circuit of the switching power supply. The switching power supply comprises, in the primary side thereof, a capacitor Ci that is a power supply having a positive terminal Pi and a negative terminal Ni, a series circuit of semiconductor switches of MOSFETs Qa and Qb connected in parallel to the capacitor Ci, and a series circuit of the primary winding WP1 and a resonant capacitor Cr connected in parallel to the MOSFET Qb. The switching power supply comprises, in the secondary side thereof, rectifying diodes D1 and D2 connected to the secondary windings WS1 and WS2, respectively, and a DC output capacitor Co that is supplied with a full-wave rectified voltage and has terminals connecting to DC output terminals Po and No. The resistor Ro connected in parallel with the capacitor Co is a dummy resistor for stabilizing the output voltage in a no load period.
The circuit for controlling the switching power supply comprises: an error amplifier GA that senses a DC output voltage Vo and amplifies the error from a reference voltage, a voltage controlling oscillator VCO that receives the output from the GA, a control circuit CNT2 connected to the output of the voltage control oscillator VCO, and a driving circuit GD that converts the output from the control circuit CNT2 to the driving signal for the MOSFETs Qa and Qb. The MOSFETs Qa and Qb of this switching power supply repeat turning ON and OFF alternately in a duty factor near 50% with a certain dead time in which the both MOSFETs are in an OFF state. Thus, current resonance operation is performed with a leakage inductance between the primary winding WP1 and the secondary windings WS1 and WS2 of the transformer T and the resonance capacitor Cr to transfer electric power from the primary side to the secondary side.
The output from the secondary winding of the transformer T is rectified by the diodes D1 and D2, and smoothed by the smoothing capacitor Co to become a DC output voltage with a small ripple. The output voltage is sensed by the error amplifier circuit GA; the voltage controlling oscillator circuit VCO controls the oscillation frequency based on the output voltage; and the control circuit CNT2 and the driving circuit GD generate the signals for ON-OFF controlling the two MOSFETs Qa and Qb alternately. Thus, stable output voltage is obtained. The switches Qa and Qb in the switching power supply repeat ON and OFF operation alternately in a duty factor near 50% with a certain dead time in which both switches are in an OFF state. Thus, a current resonant operation is performed with a leakage inductance between the primary winding WP1 and the secondary windings WS1 and WS2 of the transformer T and the resonance capacitor Cr to transfer electric power from the primary side to the secondary side.
One of the advantages of the current resonance type switching power supply is implementation of soft switching using body diodes (not shown) of the MOSFETs Qa and Qb. From the state in which the high side MOSFET Qa is in an OFF state and the low side MOSFET Qb is in an ON state carrying the current IQb in the direction indicated by the arrow in FIG. 4, when the low side MOSFET Qb turns OFF, the current IQb is commutated to the body diode Da of the high side MOSFET Qa. When an electric current is flowing through the body diode Da, the voltage Vs at the connection point between the MOSFETs Qa and Qb is nearly equal to the voltage Vi of the capacitor Ci, which is a DC power supply. As a consequence, turning ON of the MOSFET Qa in this period does not change rapidly the voltage across the MOSFET Qa. Thus, zero voltage switching (ZVS) is performed.
Similarly, when the high side MOSFET Qa is turned OFF and the current IQa that has been flowing in the MOSFET Qa is commutated to the body diode Db of the low side MOSFET Qb, and the voltage Vs at the connection point of the MOSFETs Qa and Qb becomes nearly equal to the ground potential. As a consequence, turning ON of the MOSFET Qb, in this period of current-carrying state of the body diode Db, does not change rapidly the voltage across the MOSFET Qb. Thus, zero voltage switching (ZVS) is performed in this case, too.
However, when the voltage Vs at the connection point between the MOSFETs Qa and Qb is at a certain voltage between the voltage Vi of the capacitor Ci as a DC voltage source and the ground potential, if the MOSFET Qa or MOSFET Qb is turned ON, hard switching occurs. In this case, the current through the MOSFET Qa or MOSFET Qb as well as the voltage across the MOSFET Qa or MOSFET Qb changes rapidly. This generates noise and cause power loss in the MOSFET Qa or MOSFET Qb. In addition, in the time duration the body diode Da of the MOSFET Qa is carrying an electric current, if the MOSFET Qb turns ON, through-current flows during the reverse recovery time from the DC power source Ci through the body diode Da to the MOSFET Qb. This through-current can grow instantaneously to a large current and may break down the MOSFETs Qa and Qb.
Some measures have been proposed to cope with the problems of hard switching and the through-current. Japanese Unexamined Patent Application Publication No. 2005-051918 (also referred to herein as “Patent Document 1”), for example, discloses a switching power supply in which a state of current flow through the body diode is detected by sensing the current flowing in a resonant circuit and in this state, generation of a driving signal to turn ON or OFF of the two switches is inhibited. Japanese Unexamined Patent Application Publication No. 2007-527190 (also referred to herein as “Patent Document 2”) discloses a circuit and method that copes with both problems of hard switching and through-current by directly sensing the voltage at the connection point between the two switches.
However, the structure of Patent Document 1 necessarily includes a resistor for current sensing in the resonance circuit: which causes a power loss. The structure of Patent Document 2 needs to sense a high voltage at the connection point between the two MOSFETs, which requires a control circuit that has a high voltage element, so the structure needs a large scale control circuit.
To cope with the problems, the inventor of the present invention has proposed a circuit disclosed in Japanese Patent Application No. 2011-150974 (also referred to herein as “Patent Document 3”); the circuit generating a dead time based on voltage variation sensed by an auxiliary winding provided in the transformer. FIG. 5 shows the circuit construction of the switching power supply disclosed in Patent Document 3; FIG. 6 shows the circuit construction of the voltage control oscillator VCO2 in the circuit of FIG. 5;, and FIG. 7 shows operation waveforms in the circuit of FIG. 5. The main circuit structure is similar to that of FIG. 4 except for the auxiliary winding WP2 added to the transformer T1. As shown in the circuit construction of FIG. 5, the auxiliary winding WP2 connects to a dv/dt detecting circuit DVD, the outputs P2_H and P2_L of the dv/dt detecting circuit DVD are delivered to a dead time adding circuit DT, and the output On_trig of the dead time adding circuit DT is delivered to a control circuit CNT3 and a voltage control oscillator VCO2.
FIG. 6 shows a circuit construction of the voltage control oscillator VCO2. Dead time widths, the Td1 and Td2 in FIG. 7, are determined by the circuit comprising a capacitor C2, a current source I2, a switch 52, a comparator CP2, and a reference voltage REF2. The width of the dead time is determined by the period from opening of the switch S2 at the turning OFF timing of the ON pulse until the voltage of the capacitor C2 reaches the reference voltage REF2.
The ON pulse width is determined by the integration circuit comprising a capacitor C1, a current source I1, and a switch S1. The capacitor C1 start to be charged when the dead time is passed after an On_trig is given. The ON pulse turns OFF when the voltage VC1 reaches the feedback voltage Vfb, which is the output of the error amplifier GA.
A switching frequency Fsw in the conventional current resonance type switching power supply of FIG. 4 is determined by an ON width Ton and a dead time Td determined in the voltage control oscillator VCO and given by the Formula (1) below.Fsw=1/(2*(Ton+Td))   (1)
Here, the ON width Ton is determined by the feedback voltage Vfb and the dead time Td is determined by the control circuit to be a fixed value.
A dead time Td in the conventional current resonance type switching power supply having a dead time automatic adjusting function shown in FIG. 5 is determined by a dead time automatic adjusting circuit.
Constant output voltage control uses voltage mode frequency control to perform stable operation. The ON width Ton is determined by the feedback voltage Vfb and given by the Formula (2) below.Ton=fon(Vfb)   (2)
The function fon(Vfb) is a linear or non-linear function. Therefore, the switching frequency Fsw is given by the Formula (3) below.Fsw=1/(2*(fon(Vfb)+Tdadj))   (3)
As is apparent from the Formula (3), the switching frequency Fsw is a function of the feedback voltage Vfb and the dead time Tdadj.
As shown in FIG. 7, the voltage control oscillator VCO charges the capacitor of the integrating circuit after the end of the dead time. So, variation of the dead time causes variation in the switching frequency and oscillation of resonant current. Although the feedback voltage Vfb increases linearly in the beginning of the soft starting, due to lack of feedback control, variation of the dead time Tdadj may cause oscillation and generate acoustic noise. The variation in the Tdadj needs to be absorbed in the feedback control system in normal operation, so parameter setting for phase compensation is difficult resulting in occurrence of oscillation. Thus, there is a need for an improved switching power supply in the art.